luci-app-ddns: reflect changes on ddns-scripts

* reflect updated language file for zh-cn
* change default for retry_count to 0 compatible to ddns-scripts 2.4.2-1
* addition to retry_count help description
* modified dependency on ddns-scripts 2.4.2-1

Signed-off-by: Christian Schoenebeck <christian.schoenebeck@gmail.com>